Techniques for sensing and adjusting a compliance voltage in an implantable stimulator device

ABSTRACT

Disclosed herein are methods and circuitry for monitoring and adjusting a compliance voltage in an implantable stimulator devices to an optimal value that is sufficiently high to allow for proper circuit performance (i.e., sufficient current output), but low enough that power is not needlessly wasted via excessive voltage drops across the current output circuitry. The algorithm measures output voltages across the current source and sink circuitry during at least periods of actual stimulation when both the current sources and sinks are operable, and adjusts the compliance voltage so as to reduce these output voltages to within guard band values preferably indicative for operation in transistor saturation. The output voltages can additionally be monitored during periods between stimulation pulses to improve the accuracy of the measurement, and is further beneficial in that such additional measurements are not perceptible to the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/971,685, filed Aug. 20, 2013, which was a divisional of U.S. patentapplication Ser. No. 13/446,784, filed Apr. 13, 2012 (now U.S. Pat. No.8,538,548), which was a continuation of U.S. patent application Ser. No.12/199,312, filed Aug. 27, 2008 (now U.S. Pat. No. 8,175,719), which wasin turn a continuation of U.S. patent application Ser. No. 11/305,898,filed Dec. 14, 2005 (now U.S. Pat. No. 7,444,181). Priority is claimedto each of these applications, and each is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to implantable stimulatordevices, e.g., a pulse generator used in a Spinal Cord Stimulation (SCS)system or other type of neural stimulation system. More particularly,the present invention relates to sensing and adjusting a compliancevoltage used by the output current source/sink circuitry to ensureproper circuit performance while saving power.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a spinal cord stimulation system, such as thatdisclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4,2003 in the name of inventors Paul Meadows et al., which is incorporatedherein by reference in its entirety.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. A Spinal Cord Stimulation (SCS)system typically includes an Implantable Pulse Generator (IPG) orRadio-Frequency (RF) transmitter and receiver, electrodes, at least oneelectrode lead, and, optionally, at least one electrode lead extension.The electrodes, which reside on a distal end of the electrode lead, aretypically implanted along the dura of the spinal cord, and the IPG or RFtransmitter generates electrical pulses that are delivered through theelectrodes to the nerve fibers within the spinal column. Individualelectrode contacts (the “electrodes”) are arranged in a desired patternand spacing to create an electrode array. Individual wires within one ormore electrode leads connect with each electrode in the array. Theelectrode lead(s) exit the spinal column and generally attach to one ormore electrode lead extensions. The electrode lead extensions, in turn,are typically tunneled around the torso of the patient to a subcutaneouspocket where the IPG or RF receiver is implanted. Alternatively, theelectrode lead may directly connect with the IPG or RF receiver. Forexamples of other SCS systems and other stimulation system, see U.S.Pat. Nos. 3,646,940 and 3,822,708, which are hereby incorporated byreference in their entireties. Of course, implantable pulse generatorsare active devices requiring energy for operation, such as is providedby an implanted battery or an external power source.

An IPG may include one or more output current sources/sinks that areconfigured to supply/receive stimulating current to/from the electrodeson the IPG, and ultimately to/from the patient's tissue. For example,FIG. 1 shows a basic output current source 500 and a correspondingoutput current sink 501 used to stimulate tissue, exemplifiedgenerically as a load 505 (R). As one skilled in the art willunderstand, transistors M1 and M3 in the output current source 500, andtransistors M2 and M4 in the output current sink 501, comprise a currentmirror. The current mirrors operates to mirror a reference current,I_(ref), in the output stage of the current source or sink, i.e.,I_(out)=I_(ref). The reference current I_(ref) can also be scaled byproviding paralleled numbers (M) of output transistors (i.e., M1 andM2), such that I_(out)=M*I_(ref). Selection of the various currentsources or sinks is typically provided by selection transistors 513 and513′. As already alluded to, an IPG typically has several electrodes,and the various current sources and sinks can be controlled to source orsink current to any particular electrode, E, as is efficacious fortreating a particular patient. As shown in FIG. 1, the current source500 is connected to IPG electrode E_(X) while the current sink isconnected to electrode E_(Y).

The output current sources and sinks 500, 501, as one can notice fromFIG. 1, are typically formed of transistors of differing polarities.Thus, the sources 500 are formed from P-channel transistors, while thesinks 501 are formed from N-channel transistors. Without a fulldiscussion of transistor physics, one skilled will recognize that use oftransistors of such polarities is sensible, given that the sources 500are typically tied to a positive voltage (V+, referred to herein as the“compliance voltage”), while the sources 501 are typically tied to amore negative voltage, such as ground. (A “ground voltage” as usedherein should be understood as any reference voltage with respect to thecompliance voltage). (The substrate connection (not shown) for thetransistors would typically be tied to the appropriate power supply,either V+ or ground, but could also be tied to the transistors'sources). Because the current sources and sinks 500 and 501 aregenerally digitally controllable as will be seen (e.g., by transistors513, 513′), to produce output currents I_(out) of a desired amplitude,such current sources and sinks are typically referred to asDigital-to-Analog Converter circuitry, or “DAC” circuitry. Morespecifically, in reference to the polarity of the transistors in each,the current sources 500 are typically referred to as “PDACs,” while thecurrent sinks 501 are typically referred to as “NDACs.”

Different output source/sink architectures can be used in an IPG, andare shown in FIGS. 2-4 respectively. The architecture shown in FIGS.2A-2B is disclosed in U.S. Pat. No. 6,181,969, which is incorporatedherein by reference in its entirety. As shown in FIG. 2A, in thearchitecture of the '969 patent, each electrode E_(X) has its owndedicated PDAC and NDAC circuitry, which allows that electrode to eitheroperate as a source of sink of current, or neither. As shown, the PDAC(current source) associated with electrode E₂ is active, while the NDAC(current sink) associated with electrode E₃ is active, thus producingthe current path shown. FIG. 2B shows the PDAC circuitry for aparticular electrode useable in the architecture of FIG. 2A. (Only thePDAC circuitry is shown, but one skilled in the art will recognize thatthe NDAC circuitry for a given electrode would be similarly formed ofN-channel devices). As shown, and as one skilled will appreciate,selection transistors 513 are used to digitally set the amplitude of thecurrent to be sourced at electrode E_(X) (i.e., electrode E₂ of FIG. 2A)from I_(ref) to 127 I_(ref) in increments of I_(ref). As this isexplained in detail in the above-incorporated '969 patent, it is notfurther discussed.

The current architecture of FIGS. 3A-3B is disclosed inabove-incorporated U.S. Pat. No. 6,516,227. This architecture is similarto that of FIGS. 2A-2B in that a number of discrete PDAC current sourcecircuitry blocks and NDAC current sink circuitry blocks are provided.However, the PDACs and NDACs are not dedicated to any particularelectrode, and instead, each PDAC and NDAC can be coupled to any givenelectrode via a low-impedance switching matrix, which in realitycontains a number of switches to accomplish this task.

Another current sourcing and sinking architecture is disclosed in U.S.patent application Ser. No. 11/177,503, filed Jul. 8, 2005, which isalso incorporated herein by reference in its entirety, which issummarized with respect to FIGS. 4A-4C. In this architecture, there isnot a discrete plurality of PDAC and NDAC circuit blocks to service thevarious electrodes. Instead, the current source and sink circuitry iseffectively distributed such that they can service any of theelectrodes. Thus, a master reference current I_(ref) (which can bescaled from another reference current I₁ using a DAC 407 as shown) isused as the input to a number of scalable current mirrors 410 (FIG. 4B).Any one of the current mirrors 410 can be chosen to participate in thecurrent produced at a particular electrode E_(X) via a switch block 405.Thus, there is a switch block 405 associated with each current mirror410, in which each switch block has a switch S_(X) to allow the currentfrom the associated current mirror to be passed to a particularelectrode E_(X).

Regardless of the current source/sink architecture used, all generallyhave similar current output path characteristics. That is, and referringagain to FIG. 1, the current output paths in each architecturecomprises, at a minimum, a current source output transistor (ortransistors if paralleled for current gain) (M1), a selection transistorto control the flow of the current mirror output transistor(s) (513),the load (R), a current sink mirror transistor or transistors (M2), anda selection transistor to control the flow of the current sink mirrortransistor(s) (513′). Each of these elements has some resistance, andhence some amount of the compliance voltage, V+, will be dropped acrossthese elements when current is flowing to stimulate the load, R.Specifically, the compliance voltage V+ will equalV_(DS1)+V_(R)+V_(DS2), where V_(DS1) comprises the drain-to-sourcevoltage drop across output transistor(s) M1 and selection transistor513, V_(DS2) comprises the drain-to-source voltage drop across outputtransistor(s) M2 and selection transistor 513′, and V_(R) equals thevoltage drop across the load.

Notice that the M1/M3 and M2/M4 current mirrors require that transistorsM1 and M2 operate in a saturation mode, such that the channels of thetransistors are in “pinch off.” When in saturation mode, the outputcurrent I_(out) is proportional to the gate voltage of the transistorsM1 or M2, but does not depend upon the drain voltage to the first order.However, to keep the transistors M1 and M2 in the saturation mode, acertain drain-to-source voltage, V_(DS), has to be satisfied for eachtransistor. Specifically, V_(DS) must be greater than the gate-to-sourcevoltage (V_(GS)) minus the threshold voltage (V_(T)) of the transistor(i.e., V_(DS)>V_(GS)−V_(T)). This saturation condition is necessarilysatisfied because V_(DS)=V_(GS) by virtue of the common gate/drainconnection of transistors M3 and M4. The minimum drain-to-source voltageV_(DS) that satisfies this relationship and allows transistors M1 and M2to operate in the saturation mode is typically on the order of a volt.

What this means in the context of the output current circuitry of FIG. 1is that the circuit can operate properly over a range of compliancevoltages, V+. For example, suppose a suitable therapy for a patientsuggests that a current of I_(out)=5 mA should be passed betweenelectrodes E_(X) and E_(Y) on the IPG. Suppose further that the load Requals 800 ohms. When the current of 5 mA is passed through the load, avoltage V_(R)=4V will build up across the load (V=I*R). Suppose furtherfor simplicity that the minimum drain-to-source voltage to keep theoutput transistors M1 and M2 in saturation equals 1V when the effects ofthe selection transistors 513, 513′ are included. (The actual value canbe different, but is chosen as 1V for ease of illustration). To providethis current, a minimum compliance voltage, V+ of at least 6V would beneeded; if V+<6V, the circuitry will be unable to produce the desiredamount of current.

However, the compliance voltage V+ could be higher than 6V while stillproducing the proper amount of current. For example, suppose for thesame example that the compliance voltage V+ is 8V. In this case, thecircuitry is still capable of providing the 5 mA current, and the load(which doesn't change) will still drop 4V at that current. What thismeans is that the remainder of the compliance voltage must be droppedacross the output transistors M1 and M2 as well as their associatedselection transistors 513 and 513′, e.g., 2V if the source and sink arematched.

However, running the circuit in this example with an 8V compliancevoltage is not efficient. While circuit performance is the same at both6V and 8V, i.e., both are capable of generating a 5 mA current, theformer will draw only 30 mW of power (P=I*V), while the latter will draw40 mW of power. In other words, 10 mW of power are needlessly droppedacross the output transistors M1, M2 and their selection transistors 513and 513′. This waste of power is regrettable in the context of animplantable medical device such as an IPG. As noted earlier, an IPGtypically runs from a battery, and therefore it is important to minimizecircuit operation that would otherwise needlessly drain the battery andcause the IPG to cease functioning, or needlessly require the patient tomore frequently recharge the battery.

Unfortunately, it is difficult to design the compliance voltage to anoptimal level. Depending on the electrodes stimulated, the magnitude ofcurrent required for efficient therapy for a given patient, and theresistance of the patient's flesh, an optimal compliance voltage fromthe vantage point of power conservation is variable.

Accordingly, the implantable stimulator art, or more specifically theIPG or SCS system art, would be benefited by techniques for sensing andadjusting the compliance voltage in a manner respectful of the poweravailable to the device. Such solutions are provided herein.

SUMMARY

Disclosed herein are methods and circuitry for compliance voltagesensing and adjustment in an implantable stimulator device. The presentinvention measures the voltage across (at least) both the output of thePDACs and NDACs involved in sourcing and sinking the stimulationcurrent. Specifically, the voltages across the output transistors ofactive PDACs and NDACs involved during stimulation (and, preferably,their selection transistors) are measured during actual stimulation, andpossibly during inactive periods as well and as discussed further below.These measured voltages are processed in accordance with an algorithm,where they are compared to a range of permissible guard band voltagesfor both the PDAC and NDAC outputs (e.g., 1.2 to 1.8V for the NDACoutputs, and 1.5 to 2.1V for the PDAC outputs). These guard band voltageranges comprise a range in which the output transistors are deemed to beproperly in saturation, but not excessively so.

Should the measured voltages across the output of the PDAC or NDAC beoutside of the guard band voltage, the compliance voltage is changedaccordingly to a disclosed algorithm to attempt to bring such measuredvoltages within acceptable limits while still keeping the NDACs andPDACs balanced. In a preferred embodiment, the compliance voltage V+starts at a maximum value (e.g., 16.8 V), and the PDAC and NDAC outputvoltages are measured. V+ is adjusted downward until the minimum voltageacross an active NDAC (Min(V_(NX))) is below a maximum guard bandvoltage for the output of the NDACs, e.g., 1.8V. In general, V+ is onlydecreased until this condition is reached, although the compliancevoltage can also be increased slightly (if possible) if Min(V_(NX))falls below the minimum guard band voltage (e.g., 1.2V).

Assuming Min(V_(NX)) is within the NDAC guard band (i.e., between 1.2and 1.8V), the voltage across the PDACs are similarly measured, and thecompliance voltage is potentially further decreased. Thus, if theminimum voltage across an active PDAC (Min(V_(PY))) is greater than amaximum guard band voltage for the output transistors of the PDACs,e.g., 2.1V, the compliance voltage is lowered until Min(V_(PY)) is below2.1 V. Once Min(V_(PY)) is within the PDAC guard band (i.e., between 1.5and 2.1V), the compliance voltage V+ is deemed optimal, as both theoutput voltage for the PDAC and NDAC are within the guard band voltage,and accordingly are deemed to be in saturation, but not excessively so.However, if Min(V_(PY)) is below the minimum PDAC guard band value,e.g., 1.5V, V+ can be increased (if possible) and, the compliancevoltage V+ is deemed optimal.

During this algorithm, note that V+ can be decreased if Min(V_(PY)) isabove 2.1V, even when Min(V_(NX)) is below 1.8V and otherwise isoptimal. While this would seem to run the risk of adjusting the NDACsout of alignment, note that Min(V_(NX)) is tied to (i.e., balanced with)Min(V_(PY)) by virtue of the current-voltage characteristics of bothDACs. Because the currents must match for the NDACs and the PDACs, it isdifficult to decrease Min(V_(NX)) significantly below the minimum NDACguard band threshold (e.g., 1.2V) without also bringing Min(V_(PY))below the minimum PDAC guard band voltage (e.g., 1.5V) and vice versa.Hence, due to this balancing, the compliance voltage can be reducedwithout significant risk of impacting circuit performance, i.e., suchthat the circuitry is unable to producing an optimal current.

In a further preferred embodiment, additional NDAC and PDAC outputvoltages measurements are made during periods in which actualstimulation is not occurring to further improve the accuracy of thecompliance voltage adjustment algorithm. Specifically, as well asmeasuring the output voltages of both the NDACs and the PDACs whileactive, i.e., during actual stimulation, the output of each specifiedNDAC and PDAC is also measured while no current is flowing. Thus, thevoltage across each NDAC and PDAC is measured during an interphaseperiod, with all other specified NDACs and PDACs disconnected from thecircuit via disabling of their selection transistors. This additionalnon-active, interphase measurement provides an additional output voltagespecific for each NDAC and PDAC, which, while generally 0V, may comprisea small voltage (e.g., 0.2V). When this additional measurement is usedin the algorithm, the non-active output voltage measurement for aparticular NDAC or PDAC is subtracted from the active output voltagemeasurement for that NDAC or PDAC to arrive at a difference voltage.This difference voltage, which normally would not vary significantlyfrom the active output voltage measurement, is used by the algorithm,and its assessment of voltages within the guard band, etc., to furtherimprove the algorithm's accuracy.

While such non-active measurements are not needed in useful all usefulembodiments, and while only active measurements can be used, thenon-active measurements, as well as improving accuracy, are beneficialin that they are not taken during periods of stimulation. That is tosay, such non-active measurements do not result in significant currentflow through the patient. As a result, such non-active measurements arenot perceptible by the patient. This is beneficial, because measurementscompliance-voltage-optimization measurements preferably do not involveoutput current stimulation that is not related to conditions deemednecessary for patient therapy. Thus, such additional, non-activemeasurements can improve the accuracy of compliance voltage adjustmentwithout affecting a therapy regimen prescribed by the IPG.

In short, through the use of the disclosed exemplary algorithm, anoptimal compliance voltage taking balancing of the NDACs and PDACs intoconsideration. The result is sufficient operation of the PDAC and NDACcircuitry, with as low a compliance voltage as possible. As noted above,use of the lowest compliance voltage saves power in the IPG.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will be moreapparent from the following more particular description thereof,presented in conjunction with the following drawings wherein:

FIG. 1 shows an exemplary output current source and a correspondingoutput current sink each having current digital-to-analog current (DAC)circuitry in series with a load.

FIGS. 2A-2B shows a prior art architecture for coupling output currentsources and sinks to a plurality of electrodes using hard-wireddedicated circuitry at each electrode.

FIGS. 3A-3B shows a prior art architecture for coupling output currentsources and sinks to a plurality of electrodes using a switching matrix.

FIGS. 4A-4C shows a prior art architecture for sourcing a sinkingcurrent to a plurality of electrodes using distributed current sourceand current sink circuitry.

FIG. 5 shows a block diagram that illustrates exemplary implantable,external, and surgical components of a spinal cord stimulation (SCS)system that employs an implantable stimulator device in accordance withthe present invention.

FIG. 6 shows various components of the SCS system of FIG. 5.

FIGS. 7A and 7B show the electrode array 110 and the manner in which itis coupled to the implantable stimulator device in the SCS system ofFIGS. 5 and 6.

FIG. 8 shows a block diagram that illustrates the main components of oneembodiment of an implantable stimulator device in which the inventioncan be used.

FIG. 9 shows a block diagram that illustrates another embodiment of animplantable stimulator device in which the invention can be used.

FIG. 10 shows an embodiment of circuitry useable in an implantablestimulator device for monitoring the output voltages across the currentsource and sink circuitry, and adjusting the compliance voltageaccordingly, in accordance with an embodiment of the invention.

FIG. 11 shows an example of various timing channels useable in animplantable stimulator device, and shows whether each electrode in achannel operates as a source or sink of current.

FIG. 12 shows the I-V characteristics of the output transistors ineither the current source or sink circuitry, and shows an optimal guardband voltage range within which the output transistors preferablyoperate.

FIG. 13 shows further details of the circuitry of FIG. 10.

FIG. 14 shows the active current sources and sinks for an exemplarytiming channel and the resistive network (i.e., load) which suchcircuitry stimulates.

FIGS. 15A-18B show activation of the current source/sink circuitry formeasuring various output voltages in the current source and sinkcircuitry in accordance with an embodiment of the invention.

FIGS. 19 and 20 respectively show exemplary timing signals indicative ofthe operation of the compliance voltage monitoring circuitry of FIGS. 10and 13 for the active current sinks (NDACs) and current sources (PDACs).

FIG. 21 shows a flow chart illustrating an exemplary algorithm formonitoring and adjusting the compliance voltage in accordance with anembodiment of the invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims and their equivalents.

Before discussing the compliance voltage sensing and adjustment aspectsof the invention, the circuitry, structure, and function of animplantable stimulator device in which the technique can be used is setforth for completeness. The disclosed implantable stimulator device maycomprise implantable pulse generator (IPG), or similar electricalstimulator and/or electrical sensor, that may be used as a component ofnumerous different types of stimulation systems. More specifically, thedescription that follows relates to use of the invention within a spinalcord stimulation (SCS) system as an exemplary embodiment. However, it isto be understood that the invention is not so limited. Rather, theinvention may be used with any type of implantable electrical circuitrythat could benefit from improved compliance voltage monitoring andadjustment. For example, the present invention may be used as part of apacemaker, an implantable pump, a defibrillator, a cochlear stimulator,a retinal stimulator, a stimulator configured to produce coordinatedlimb movement, a cortical or deep brain stimulator, or in any otherstimulator configured to treat urinary incontinence, sleep apnea,shoulder sublaxation, etc. Moreover, the technique can be used innon-medical and/or non-implantable devices or systems as well, such as aTranscutaneous Electrical Nerve Stimulator (TENS) device.

Turning first to FIG. 5, a block diagram is shown that illustrates thevarious components of an exemplary SCS system in which the invention maybe used. These components may be subdivided into three broad categories:implantable components 10, external components 20, and surgicalcomponents 30. As seen in FIG. 5, the implantable components 10 includean implantable pulse generator (IPG) 100, an electrode array 110, and(as needed) a lead extension 120. The extension 120 may be used toelectrically connect the electrode array 110 to the IPG 100. In anexemplary embodiment, the IPG 100, described more fully below, maycomprise a rechargeable, multi-channel, telemetry-controlled, pulsegenerator housed in a rounded high-resistivity titanium alloy case 116(FIG. 7A) to reduce eddy current heating during the inductive chargingprocess. The IPG 100 may provide electrical stimulation through amultiplicity of electrodes, e.g., sixteen electrodes, included withinthe electrode array 110, as discussed further below with reference toFIGS. 7A and 7B.

Typically, the IPG 100 is placed in a surgically-made pocket either inthe abdomen, or just at the top of the buttocks. It may, of course, alsobe implanted in other locations of the patient's body. Once implanted,the IPG 100 is detachably connected to the lead system, comprising thelead extension 120, if needed, and the electrode array 110. The leadextension 120, for example, may be tunneled up to the spinal column.Once implanted and any trial stimulation period is complete, the leadsystem 110 and lead extension 120 are intended to be permanent. Incontrast, the IPG 100 may be replaced when its power source fails or forother reasons.

As seen best in FIG. 6, and as also illustrated in FIG. 5, the electrodearray 110 and its associated lead system typically interface with theimplantable pulse generator (IPG) 100 via the lead extension system 120just mentioned. The electrode array 110 may also be connected to anexternal trial stimulator 140, through the use of a percutaneous leadextension 132 and/or an external cable 134. The external trialstimulator 140 typically includes the same or similar pulse generationcircuitry as does the IPG 100, and is used on a trial basis, e.g., for7-10 days, after the electrode array has been implanted and prior toimplantation of the IPG 100, to test the effectiveness of thestimulation that is to be provided.

FIGS. 7A and 7B show the electrode array 110 and the manner in which itis coupled to the IPG 100. As shown, the electrode array 110 comprisesfirst and second implantable leads 102 and 104. Leads 102 and 104 arein-line leads, meaning that both consist of a plurality of in-lineelectrodes 106. The electrodes are carried on a flexible body 108. Inthe illustrated embodiment, there are eight electrodes on lead 102,labeled E₁-E₈, and eight electrodes on lead 104, labeled E₉-E₁₆. Theactual number of leads and electrodes will, of course, vary according tothe intended application and should not be understood in any limitingsense. As discussed above, leads 102 and 104 may be implanted into adesired location, such as adjacent to the patient's spinal column,through the use of an insertion needle or other conventional techniques.

Each of the electrodes 106 on lead 102 are electrically connected to theIPG 100 by a first signal wire 112 that extends through, or is imbeddedin, the associated flexible body 108. Similarly, each of the electrodes106 on the lead 104 are electrically connected to the IPG 100 by secondsignal wires 114. The signal wires 112 and 114 are connected to the IPG100 by way of an interface 115. The interface 115 may be any suitabledevice that allows the leads 102 and 104 (or a lead extension 120, notshown) to be removably connected to the IPG 110. Interface 115 maycomprise, for example, an electro-mechanical connector arrangementincluding lead connectors 117 a and 117 b (FIG. 7A) configured to matewith corresponding connectors (only connector 119 a is shown) on theleads 102 and 104. Alternatively, the leads 102 and 104 can share asingle connector that mates with a corresponding connector on the IPG100. Exemplary connector arrangements are disclosed in U.S. Pat. Nos.6,609,029 and 6,741,892, which are incorporated herein by reference.

Referring again to FIGS. 5 and 6, and as noted earlier, a hand-heldprogrammer (HHP) 202 may be used to control the IPG 100 via a suitablenon-invasive communications link 201, e.g., an RF link. Such controlallows the IPG 100 to be turned on or off, and generally allowsstimulation parameters, e.g., pulse amplitude, width, and rate, to beset by a patient or clinician within prescribed limits. The HHP 202 mayalso be linked with the external trial stimulator 140 through anotherlink 205′, e.g., an infra red link. Detailed programming of the IPG 100is preferably accomplished through the use of an external clinician'sprogrammer (CP) 204 (FIG. 5), which may also be hand-held and which maybe coupled to the IPG 100 directly via link 201 a or indirectly throughthe HHP 202. An external charger 208, non-invasively coupled with theIPG 100 through link 209, e.g., an inductive link, allows energy storedor otherwise made available to the charger 208 to be coupled into therechargeable battery housed within the IPG 100.

Turning next to FIG. 8, a block diagram is shown that illustrates themain components of one embodiment of an implantable pulse generator(IPG) 100 in which embodiments of the invention may be used. As seen inFIG. 8, the IPG includes a microcontroller (X) 160 connected to memorycircuitry 162. The μC 160 typically comprises a microprocessor andassociated logic circuitry, which in combination with control logiccircuits 166, timer logic 168, and an oscillator and clock circuit 164,generate the necessary control and status signals to allow the μC 160 tocontrol the operation of the IPG in accordance with a selected operatingprogram and stimulation parameters.

The operating program and stimulation parameters are telemetered to theIPG 100, where they are received via antenna 250 (which may include acoil 170 and/or other antenna components), processed, e.g., viaRF-telemetry circuitry 172, and may be stored, e.g., within the memory162. The RF-telemetry circuitry 172 demodulates the signal it receivesfrom the HHP 202 or CP 204 to recover the operating program and/or thestimulation parameters. More specifically, signals received by theantenna 250 are passed through the transmit/receive switch 254 toamplifiers and filters 258. From there, the received signals aredemodulated (262) using Frequency Shift Keying (FSK) demodulation forexample, and the data is then sent to the microcontroller 160 forprocessing and/or eventual storage. When RF-telemetry circuitry 172 isused to transmit information to the HHP 202 or CP 204 to report in somefashion on its status, the microcontroller 160 sends relevant data totransmission drivers 256, where the carrier is modulated by the data andamplified for transmission. The transmit/receive switch 254 would thenbe set to communicate with the transmission drivers 256, which in turndrive the data to the antenna 250 to be broadcast.

The microcontroller 160 is further coupled to monitoring circuits 174via bus 173. The monitoring circuits 174 monitor the status of variousnodes or other points 175 throughout the IPG 100, e.g., power supplyvoltages, current values, temperature, the impedance of electrodesattached to the various electrodes E₁ . . . E_(N), and the like.Informational data sensed through the monitoring circuit 174 may be sentto a remote location external to the IPG (e.g., a non-implantedlocation) through telemetry circuitry 172 via coil 170. Further detailsconcerning the monitoring circuitry 174 will be discussed later in thisdisclosure.

The operating power for the IPG 100 may be derived from a rechargeablepower source 180, which may comprise a lithium-ion or lithium-ionpolymer battery, for example. The rechargeable battery 180 provides anunregulated voltage to power circuits 182. The power circuits 182, inturn, generate the various voltages 184, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 100. In a preferred embodiment, the battery 180 is charged by anelectromagnetic field created by an external portable charger 208 (FIG.5). When placed near the IPG 100 (e.g., centimeters away), anelectromagnetic field emanating from the portable charger 208 induces acurrent in charging coil 270 (even through a patient's skin). Thiscurrent is then rectified and regulated to charge the battery 180.Further associated with the charging circuitry is charging telemetrycircuitry 272, which is used for example by the IPG 100 to report backto the portable charger 208 when the battery is full, and thus whenportable charger can be shut off.

In one exemplary embodiment, any of the N electrodes may be assigned toup to k possible groups or “channels.” In one preferred embodiment, kmay equal four. Moreover, any of the N electrodes can operate, or beincluded in, any of the k channels. The channel identifies whichelectrodes are selected to synchronously source or sink current tocreate an electric field in the tissue to be stimulated. Amplitudes andpolarities of electrodes on a channel may vary, e.g., as controlled bythe HHP 202 and/or the CP 204.

For example, as shown in FIG. 11, four channels are defined, andrepresent groups of electrodes that will be activated as either sourcesor sinks at a particular time. Thus, in a first timing channel A,electrodes E₁ and E₄ will act as current sources (denoted by the plussymbol), while electrodes E₃ and E₅ will act as sinks (denoted by theminus symbol). Electrodes without any designator in FIG. 11 are notactivated and do not participate in the sourcing or sinking of current.By designating different channels in this manner, the stimulationprovided to the patient can be freely varied with desired therapeuticeffect. Note that the case 116 (FIG. 7A) of the IPG 100 can also operateas an electrode which can source or sink current. This allows the IPG tobe operated in any number of different modes, e.g., a monopolar mode(one electrode E_(X) active with an active case), a bipolar mode (twoelectrodes E_(X) active), or a multipolar mode (more than two electrodesE_(X) active).

Ultimately, the grouping of the electrodes into different channels ismanaged by the control logic 166 (FIG. 8), with the timing of theactivation of the various electrodes in each channel being handled bythe timer logic 168. The control logic 166, receiving commands from themicrocontroller 160, further sets the amplitude of the current pulsebeing sourced or sunk to or from a given electrode contact. Such currentpulse may be programmed to one of several discrete current levels, e.g.,between 0 to 10 mA in steps of 0.1 mA. The pulse width of the currentpulses is preferably adjustable in convenient increments, e.g., from 0to 1 milliseconds (ms) in increments of 10 microseconds (μs). Similarly,the pulse rate is preferably adjustable within acceptable limits, e.g.,from 0 to 1000 Hz. Other programmable features can include slowstart/end ramping, burst stimulation cycling (on for X time, off for Ytime), and open or closed loop sensing modes.

The stimulation pulses generated by the IPG 100 may be charge balanced.This means that the amount of positive charge associated with a givenstimulus pulse is offset with an equal and opposite negative charge.Charge balance may be achieved through coupling capacitors C_(X), whichprovide a passive capacitor discharge that achieves the desiredcharge-balanced condition. Alternatively, active biphasic ormulti-phasic pulses with positive and negative phases that are balancedmay be used to achieve the needed charge balanced condition.

As shown in FIG. 8, much of circuitry included within the IPG 100 may berealized on a single application specific integrated circuit (ASIC) 190.This allows the overall size of the IPG 100 to be quite small, andreadily housed within a suitable hermetically-sealed case 116 (FIG. 7A).The IPG 100 may include feedthroughs to allow electrical contact to beindividually made from inside of the hermetically-sealed case with the Nelectrodes that form part of the lead system outside of the case, as wasdiscussed above with reference to FIG. 7B.

The telemetry features of the IPG 100 allow the status of the IPG to bechecked as noted earlier. For example, when the HHP 202 and/or the CP204 initiate a programming session with the IPG 100 (FIG. 5), thecapacity of the battery is telemetered so that the external programmercan calculate the estimated time to recharge. Any changes made to thecurrent stimulus parameters are confirmed through back-telemetry,thereby assuring that such changes have been correctly received andimplemented within the implant system. Moreover, upon interrogation bythe external programmer, all programmable settings stored within theimplant system 10 may be uploaded to one or more external programmers.

Turning next to FIG. 9, a hybrid block diagram of an alternativeembodiment of an IPG 100′ that may be used with the invention isillustrated. The IPG 100′ includes both analog and digital dies, orintegrated circuits (ICs), which may be housed in a singlehermetically-sealed rounded case having, for instance, a diameter ofabout 45 mm and a maximum thickness of about 10 mm. Many of the circuitscontained within the IPG 100′ are identical or similar to the circuitscontained within the IPG 100, shown in FIG. 8. The IPG 100′ includes aprocessor die, or chip, 160′, an RF telemetry circuit 172′ (typicallyrealized with discrete components), a charger coil 270′, a rechargeablebattery 180′, battery charger and protection circuits 272′, 182′, memorycircuits 162′ (SEEPROM) and 163′ (SRAM), a digital IC 191′, an analog IC190′, and a capacitor array and header connector 192′.

The capacitor array and header connector 192′ include sixteen outputdecoupling capacitors, as well as respective feed-through connectors forconnecting one side of each decoupling capacitor through thehermetically-sealed case to a connector to which the electrode array110, or lead extension 120, may be detachably connected.

The processor 160′ may be realized with an application specificintegrated circuit (ASIC), field programmable gate array (FPGA), or thelike that comprises a main device for full bi-directional communicationand programming. The processor 160′ may utilize an 8086 core (the 8086is a commercially-available microprocessor available from, e.g., Intel),or a low power equivalent thereof, SRAM or other memory, two synchronousserial interface circuits, a serial EEPROM interface, and a ROM bootloader 735. The processor die 160′ may further include an efficientclock oscillator circuit 164′, and (as noted earlier) mixer andmodulator/demodulator circuitry implementing the QFAST RF telemetrymethod. An analog-to-digital converter (A/D) circuit 734 is alsoresident on the processor 160′ to allow monitoring of various systemlevel analog signals, impedances, regulator status and battery voltage.The processor 160′ further includes the necessary communication links toother individual ASICs utilized within the IPG 100′. The processor 160′,like all similar processors, operates in accordance with a program thatis stored within its memory circuits.

The analog IC (AIC) 190′ may comprise an ASIC that functions as the mainintegrated circuit that performs several tasks necessary for thefunctionality of the IPG 100′, including providing power regulation,stimulus output, and impedance measurement and monitoring. Electroniccircuitry 194′ performs the impedance measurement and monitoringfunction.

The analog IC 190′ may also include output current DAC circuitry 186′configured to supply current to a load, such as tissue, for example. Theoutput current DAC circuitry 186′ may be configured to deliver up to 20mA aggregate and up to 12.7 mA on a single channel in 0.1 mA steps.However, it will be noted that the output current DAC circuitry 186′ maybe configured to deliver any amount of aggregate current and any amountof current on a single channel, according to one exemplary embodiment.

Regulators for the IPG 100′ supply the processor and the digitalsequencer with a voltage. Digital interface circuits residing on theanalog IC 190′ are similarly supplied with a voltage. A programmableregulator supplies the operating voltage for the output current DACcircuitry 186′. The coupling capacitors C_(X) and electrodes E_(X), aswell as the remaining circuitry on the analog IC 186′, may all be housedwithin the hermetically sealed case of the IPG 100. A feedthrough pin,which is included as part of the header connector 192′, allowselectrical connection to be made between each of the coupling capacitorsC_(N) and the respective electrodes E₁, E₂, E₃, . . . , or E₁₆.

The digital IC (DigIC) 191′ functions as the primary interface betweenthe processor 160′ and the output current DAC circuitry 186′, and itsmain function is to provide stimulus information to the output currentDAC circuitry 186′. The DigIC 191′ thus controls and changes thestimulus levels and sequences when prompted by the processor 160′. In anexemplary embodiment, the DigIC 191′ comprises a digital applicationspecific integrated circuit (digital ASIC).

With the basic structure of an implantable stimulator understood, focusnow shifts to a detailed description of the compliance voltage sensingand adjustment techniques that are the focus of this disclosure.

As noted earlier, the compliance voltage, V+, can be set to variousvalues while still exhibiting satisfactory current sourcing/sinkingperformance. Thus, the NDACs (current sinks) and the PDACs (currentsources) involved in stimulation of tissue can be powered by acompliance voltage ranging from a minimum value (below which currentwill be too low) to any maximum value which the IPG 100 is capable ofproviding. Within this range, the stimulation current desired by aparticular therapeutic regimen can be provided. However, while thecompliance voltage V+ can vary over a range of values while exhibitingsatisfactory performance, power is needlessly lost should the compliancevoltage be set to a value that is too high. Specifically, if thecompliance voltage is set too high, the drain-to-source voltage (V_(DS))across the output transistors 502, 503 (FIG. 1) is needlessly increasedbeyond the saturation values that are required for proper circuitoperation. The result, as noted earlier, is needlessly wasted power inthe IPG 100, which reduces battery life.

Accordingly, the present invention measures the voltage across (atleast) the output of the PDACs and NDACs involved in sourcing andsinking the stimulation current. In a preferred embodiment, the voltageacross the PDAC and NDAC selection transistors as well as the PDAC andNDAC output transistors is also included in this measurement, althoughsuch additional voltage due to the selection transistors, whilesignificant, may be relatively small. These voltages are measured (atleast) during actual stimulation, and are compared to a range ofpermissible guard band voltages for both the PDAC and NDAC outputs(e.g., 1.2 to 1.8V for the NDAC outputs, and 1.5 to 2.1V for the PDACoutputs). These guard band voltage ranges comprise a range in which theoutput transistors are deemed to be properly in saturation, but notexcessively so.

Should the measured voltages across the output of the PDAC or NDAC beoutside of the guard band voltages, the compliance voltage, V+, ischanged to attempt to bring such measured voltages within acceptablelimits in accordance with an algorithm explained in further detailbelow. However, before discussing this algorithm, specifics of thecircuitry used to measure the output voltages is discussed, startingwith FIG. 10. For the most part, FIG. 10 illustrates further details ofthe monitoring circuitry 174 of FIG. 8, which comprises (in addition toother components) a compliance voltage sensing control circuit 605, aswitching matrix 143, at least one voltage sensor 600, and a compliancevoltage regulator 610. Feeding into this circuitry, and specificallyinto the switching matrix 143, are lines L 175 associated with eachelectrode. In a given architecture, more than one current source circuitor current sink circuit (e.g., current mirror) may contribute to thecurrent at a particular electrode. However, for ease of illustration,only one source circuit 500 and sink circuit 501 are shown in FIG. 10for simplicity.

Given the tap point of the lines L 175, the voltage present on the linesis indicative of the output voltage of the source and sink circuitry. Asshown, this output voltage comprises the voltage drop across both theoutput transistors of the source and sink circuitry 502, 503 (which asnoted earlier, can comprise a plurality of paralleled transistors toscale current gain), and across the selection transistors 513, 513′ usedto select those output transistors as contributing to the current. Inother embodiments, the lines could be placed between the outputtransistors 502 and 503 and the selection transistors 513, 513′,although this is not considered as beneficial because it would excludefrom monitoring realistic voltage drops occurring across the selectiontransistors 513, 513′. The lines 175 are used in the sensing of theoutput voltage. The output voltage across current sink (NDAC) 501 atelectrode E_(Y) comprises the absolute voltage on line L_(NY), whichdoes require a difference calculation as the NDAC is referenced toground. By contrast, the output voltage across the current source (PDAC)500 at electrode E_(X) comprises the difference between the compliancevoltage, V+, and the voltage measured at line L_(PX).

The voltages for the lines 175 are provided to a switching matrix 143.As noted above, while only two lines 175 are shown for ease ofillustration in FIG. 10, many more lines would be present, depending onthe number of electrodes present. In one embodiment, the switchingmatrix 143 is used to select the voltage on one line, and to presentthat voltage (L) to voltage sensor 600. As can be seen, the selectiontransistors 513, 513′, the switching matrix 143, and the voltage sensor600 are all controlled by a compliance voltage sensing control circuitry605 via busses 606, 607, and 608. Ultimately, the compliance voltagesensing circuit 605 receives signals from the microcontroller 160 (FIG.8), which informs the control circuitry 174 when and how the variousmeasurements are to be made consistent with the disclosed algorithm, asexplained in further detail below.

The voltage sensor 600, in one embodiment, outputs an analog voltage,“Out,” to the microcontroller 160, which as shown contains ananalog-to-digital (A/D) interface 635. This allows the microcontroller160 to understand and digitally process the output voltage, and inaccordance with the disclosed algorithm to send control signals to thecompliance voltage regulators 610, i.e., the circuitry that ultimatelyadjusts the compliance voltage, V+.

As noted earlier, it is preferable in a current-mirror based source orsink circuit that the output transistors 502, 503 operate in saturation,but not excessively so, lest power is needlessly lost. Accordingly,embodiments of the invention seek to adjust the compliance voltage tokeep the output transistors in active current sources and sinks insaturation. This may not always be possible, recognizing that thecurrent source and current sink circuitry are serially connected throughthe load and hence act to “balance” one another, consistent with theircurrent-voltage characteristics. If perfect saturation performancecannot be achieved in both the source and the sink, the compliancevoltage will be set to as logical a value as possible to ensure propercircuit performance with minimal power loss.

FIG. 12 shows the current-voltage characteristics for the outputtransistors 502, 503 viewed in isolation. (More accurately, the I-Vcurve for the N-channel output transistors 503 in the sinks 501 isshown; one skilled will understand that the P-channel output transistors502 in the sources would have opposite polarity as is typical in CMOScircuit design). The I-V curve exhibits a saturation voltage, Vsat,above which the drain-to-source voltage, Vds, is sufficient to cause thetransistor to operate in saturation. In a preferred embodiment, it isdesired that the output transistors 502, 503 operate within a guard bandrange of voltage as shown. While the lower limit of the guard band couldcomprise Vsat, it is preferred to choose a slightly elevated lower rangevalue to allow for margin and to account for the normally-small voltagedrop across the selection transistors 513, 513′ which may also beincluded in the monitored output voltages. The upper limit of the guardband voltage is chosen not to be excessive, and demarks a limit withinthe output transistors are in saturation, but not excessively so. Inpreferred embodiments, the guard band voltage for current sources(PDACs) ranges from 1.5V to 2.1V, while the guard band voltage forcurrent sinks (NDACs) ranges from 1.2V to 1.8V. (The lower values forthe NDAC reflect that the N-channel output transistors 503 wouldnormally have slightly lower saturation voltages than would comparableP-channel output transistors 502 in the PDAC).

As discussed earlier, FIG. 11 shows various timing channels useable byan IPG 100, and specifies which device electrodes are to act as currentsources and sinks at a particular time. Timing channel A is used toillustrate an embodiment of the invention for compliance voltageadjustment. As can be seen, in channel A, two electrodes act as sources(E₁ and E₄), while two electrodes act as sinks (E₃ and E₅). As discussedearlier, more or less electrodes can act as sources or sinks of current,although only two of each are illustrated in exemplary timing channel A.So configured, the representative circuit, including the resistivenetwork 505 constituting the patient's tissue, is shown in FIG. 14 forchannel A. As one will appreciate, the various sources 500 and sinks 501can be comprised of PDAC or NDAC circuitry dedicated to a particularelectrode within the IPG 100 (e.g., FIGS. 2A and 2B), or could compriseother architectures as discussed earlier (e.g., FIGS. 3A to 4C).

Further details of this exemplary timing channel configuration, and themeasurements made to monitor and ultimately adjust the compliancevoltage V+, and shown with respect to FIGS. 15A-18B. Note in any one ofthese Figures that the output transistors 502, 503, the selectiontransistors 513, 513′, the resistive network 505, and the lines Lassociated with each electrode, are shown. FIGS. 15A, 16A, 17A, and 18Arepresent the configuration of the source (PDAC) and sink (NDAC)circuitry during the delivery of actual stimulation pulses to thepatient as prescribed by timing channel A and other specifics ofstimulation (e.g., pulse width, amplitude, frequency, etc.). Thus, notein these Figures that all PDACS and NDACs are connected to the circuit,i.e., the selection transistors 513, 513′ are all on, as designated bythe arrow with each transistor. Note also that in each of FIGS. 15A,16A, 17A and 18A that the output voltage of the NDAC or PDAC circuitryat each electrode (V_(N3A), V_(N5A), V_(P1A), V_(P4A)) is monitored viaits corresponding line (L₃, L₅, L₁, L₄), starting first with the NDACs(lines L₃, L₅) followed by the PDACs (lines L₁, L₄).

As shown, each of these output voltages (V_(N3A), V_(N5A), V_(P1A),V_(P4A)) are monitored in series, with L₃'s output voltage beingmeasured during a first stimulation phase, L₅'s output being measuredduring a second stimulation phase, etc. However, while serial monitoringis a necessity in embodiments having only a single voltage sensor 600(FIG. 10), it should be noted that these output voltages can also bemonitored in parallel. Thus, if it is assumed that four PDACs can act ascurrent sources and four NDACs can act as current sinks during onetiming channel, then eight voltage sensors 600 (not shown) would allowfor simultaneous sensing of all voltages. In such a case, bus 607 wouldenable the switching matrix 143 to simultaneously pass these eightvoltages to the eight different voltage sensors 600.

In any event, and as noted earlier, these NDAC and PDAC output voltagesas tapped by lines 175 are passed via the switching matrix 143 to thevoltage sensor 600, for which further details are shown in FIG. 13. In apreferred embodiment, the voltage on any particular chosen line L isdeduced by sending that voltage to a difference amplifier 625. As iscommon with such difference amplifiers, a reference voltage Vref is alsoinput. Reference voltage Vref is maintained by reference voltagegenerator 615, which may comprise a 1.2V band gap reference circuit forexample. Reference voltage generator 615 is controlled by bus 614 passedfrom compliance control 605 to allow an appropriate reference voltage tobe generated with a proper value and at an appropriate time.

Ultimately, the differential voltage (Out; FIGS. 10, 13) is sent to theA/D interface 635 of the microcontroller 160. In this regard, signal“Out” may be a relative value representative of the output voltagesacross the NDACs and PDACs, but may not comprise the actual value ofthose output voltages. Instead, the actual value of the output voltagedrops may be computed in the microcontroller 160, which for example mayderive the actual output voltages by subtracting out the effects of thereference voltage, Vref; by comparing such voltages to the currently-setcompliance voltage, V+, etc. In other words, signal “Out” needs merelyto inform the microcontroller 160 of a value from which themicrocontroller can deduce the output voltage across the current sourceand sink circuitry. Of course, output voltage sensing can occur in manydifferent ways, and as one skilled in the art will appreciate. Forexample, although in a preferred embodiment sensing occurs relative to areference voltage, Vref, sensing could also occur relative to thecompliance voltage, V+, ground, or any other voltage. In short, what isimportant is that the output voltages on the lines L 175 be sensed; theway in which this specifically occurs is not important.

FIGS. 19 and 20 respectively show an embodiment of how these outputvoltages can be deduced for both the NDACs (FIG. 19) and the PDACs (FIG.20). In both Figures, the three top traces represent signals indicativeof the operation of the IPG and of the compliance voltage sensingcircuitry. The first signal indicates when active stimulation isoccurring in the IPG 100. As shown in this example, that period is 260microseconds, but this length can vary depending on the frequency andduration of the stimulation pulses specified for a given patient'stherapy, as explained further below. After the active stimulation phase,i.e., starting at 260 microseconds, the IPG enters an “interphase”period during which no stimulation occurs. (The interphase period, andhow it can be used in voltage monitoring and compliance voltageadjustment, will be explained in further detail later).

The second trace shown in FIGS. 19 and 20 comprises a control signalwhich dictates when the output voltages are monitored during the activestimulation phase. This second trace signal, called “sample duringactive,” would comprise one or more control signals sent by thecompliance voltage sensing control circuitry 605 (see FIGS. 10 and 13).As shown, the sample during active signal comprises a set up/samplesignal. Specifically, on the rising edge, the output voltage ispermitted to pass to the voltage sensor 600, for example, by allow thevoltages on lines 175 to pass through the switching matrix 143 (seeFIGS. 10 and 13). The falling edge of the sample during active signal,by contrast, actually sampled the output voltage at the voltage sensor600. In the particular embodiment illustrated, this type of setup/sample scheme is beneficial to allow the output voltages to settleand ramp to appropriate levels This is necessary when considering thesettling time and capacitance of the voltage sensing circuit, asdiscussed further below. Such voltage settling can be seen in the fourthand fifth traces, which respectively show the output voltage across theNDAC (FIG. 19) and PDAC (FIG. 20), and those voltages as input to thevoltage sensor 600. As can be seen, by allowing the passed voltages tostabilize, they can be sampled at reliable, stable values.

In the embodiment shown in FIGS. 19 and 20, set up and sampling occurtowards the end of the active stimulation period, i.e., at 240 and 259microseconds respectively. Set up and sampling is beneficial towards theend of the stimulation pulse, again, to allow for settling andstabilization of the circuitry during stimulation. For example, thecoupling capacitors, Cx, charge up during the active stimulation period,such that the voltage across them is the largest at the end of theactive stimulation pulse. Therefore, it is preferable for sampling tooccur towards the end of the active stimulation period, as this allowscompliance voltage sensing to account for any such coupling capacitorvoltages. Moreover, because the compliance voltage V+ might droop duringactive stimulation, sampling near the end of active stimulation isfurther beneficial as this will consider such a droop when it is at itsmaximum. Additionally, sensing near the end of stimulation also allowspolarization voltage build-up on the electrode-tissue interface to beaccounted for.

Of course, set up and sampling of the output voltages across the NDACsand PDACs can occur in many different way, and will be implementationspecific. Thus, just as the voltage sensor 600 can be implemented inseveral different ways, sampling of the voltage too will depend on theimplementation used. In short, the sampling and control signals as shownin FIGS. 19 and 20 should be understood as merely exemplary of oneembodiment. Again, the important issue is that the output voltage acrossthe NDACs and PDACs be monitored at logical times so these values can bepassed to and processed by the compliance voltage adjustment algorithmto be discussed shortly.

Before discussing this algorithm, an alternative embodiment formeasuring the output voltage across the NDACs and PDACs is discussed. Inthis variation, not only is the output voltage during stimulationmeasured, but the output voltage during periods of no stimulation ismeasured as well. Specifically, in a preferred embodiment, the outputvoltage is measured during the interphase period, i.e., during a periodbetween active stimulation pulses. Such measurements are shown in FIGS.15B, 16B, 17B, and 18B. As seen in those Figures, the voltage on theline of interest for each active NDAC and PDAC in a given timing channelis monitored while all other DACs are disconnected. For example, in FIG.15B, the voltage across the first NDAC, corresponding to electrode E₃,is measured (V_(N3B)), and thus its selection transistor, 513 ₃′, is on.By contrast, all other DACs are off, i.e., selection transistors 513 ₁,513 ₄, and 513 ₅′ are off. As a result, no or minimal current will flowthrough the resistive network 505 comprising the patient's flesh, as thepathway between the compliance voltage, V+, and ground is interrupted.However, even though the NDAC at electrode E₃ is disconnected from thecompliance voltage, V+, some residual output voltage may be present online L₃ by virtue of that NDAC otherwise being on.

This interphase output voltage measurement can be used to improve theaccuracy of the output voltage as processed by the microcontroller 160,and ultimately as considered by the compliance voltage adjustmentalgorithm. Preferably, the interphase output voltage measurement (again,generally negligible but potentially significant in value) is subtractedfrom the active stimulation measurement at any particular DAC. Thus,notice in FIGS. 15A and 15B that active (A) and interphase (B)measurements are made at electrode E₃, which are designated as V_(N3A)and V_(N3B), and likewise for the other DACs in FIGS. 16, 17 and 18. Theresult in this example is eight output voltage measurements: V_(N3A),V_(N5A), V_(P1A), V_(P4A) for the active measurements, and V_(N3B),V_(N5B), V_(P1B), V_(P4B) for the interphase measurements.

To improve the accuracy of the output voltages for the DACs to beprocessed by the compliance voltage adjustment algorithm, these outputactive and interphase measurements are preferably subtracted for eachDAC. Thus, in a preferred embodiment, the output voltage across the DACat electrode E₃ to be sent to the compliance adjustment voltagealgorithm would comprise V_(N3A)−V_(N3B). By doing this, assessment ofthe operating region (saturation) of the output transistors in the DACcan be improved, as residual voltages due to the active DAC itself, aswell as voltage drops resulting from the selection transistors 513, 513′can be corrected out of the output voltage measurement.

Taking of the interphase output voltage measurement is shown withreference once again to FIGS. 19 and 20. Specifically, the third trace,called “sample during interphase,” comprises a control signal whichdictates when the output voltages are monitored during the interphaseperiod. This third control signal is similar to the second controlsignal (sample during active) discussed earlier, and comprises a setup/sample signal. Specifically, after set up, the falling edge actuallysamples the input to the voltage sensor, which again requires some timeto stabilize from its active value. In the example shown in FIGS. 19 and20, such sampling occurs at 279 microseconds, i.e., 19 microseconds intothe interphase period, but again these times and sensing schemes canvary and are implementation dependent. The important issue, should aninterphase measurement be used to supplement the active measurement, isto take the measurement at a sensible time consistent with theimplementation chosen, and this can occur in different ways and withdifferent timings.

It should be noted while the monitoring of the interphase outputvoltages can be beneficial for the reasons just explained, the use ofsuch interphase measurements is not required in all useful embodiments.Instead, only the active phase measurements (i.e., those illustrated inFIGS. 15A, 16A, 17A, and 18A) can be used. As previously noted, thoseactive output voltage measurements can be made simultaneously. Bycontrast, should interphase measurements be made to measure the inactiveoutput voltage across each DAC, such measurements would need to be madeserially, i.e., during sequential interphase periods.

In a preferred embodiment, the output voltage measurements for eachactive electrode specified within a particular timing channel is takensequentially, and in the order specified in FIGS. 15A-18B. Thus, thesinking electrodes are measured first: thus the NDAC circuitryassociated with electrode E₃ is measured first during active stimulation(FIG. 15A), followed by the interphase (inactive) measurement for E₃(FIG. 15B), followed later by the same measurements for E₅ (FIGS. 16A &B), etc. The compliance voltage can then be adjusted as specified withrespect to FIG. 21 below. Then, the sourcing electrodes are similarlymeasured in series (FIG. 17A-18B).

Because the measurements are made in the context of actual prescribedtherapy, the timing of the monitoring signals is preferably adjustable.In this regard, it is particularly important to consider the frequencyand duration of the prescribed stimulation pulses so that the setup/sample measurements are properly “fit” to the stimulation pulses andto the interphase periods between them. For example, while the timing ofthe signals for the example shown in FIGS. 19 and 20 would beappropriate for one frequency, a higher frequency would require tightertiming.

Moreover, between each stimulation pulse, and as is common inimplantable stimulators, a charge recovery phase may follow. As isknown, such a recovery period could comprise pulsing to recover thecharge passed into the load, i.e., the tissue 505. Usually, suchrecovery is biphasic, and is implemented by recovery pulses of equalmagnitude and duration to the active pulses but of opposite polarity.Charge recovery can also be accomplished passively, as is well known.Such charge recovery is not shown in the Figures for simplicity, butwould be a consideration in an embodiment of the invention. Moreover, itshould be noted that DAC output voltages can be measured during chargerecovery, but again this is not further discussed for simplicity.

Now that the various means for monitoring the output voltage of the DACshas been described, attention turns to how, algorithmically, thosevoltages are used to adjust the compliance voltage to an efficientlevel. As shown in FIG. 21, the algorithm starts by first acquiring allrelevant output voltages for the NDACs (V_(N1), V_(N2), . . . V_(NX))for a given timing channel as just described. It should be understoodthat these output voltages can comprise only the measurements takenduring active stimulation (e.g., V_(N3A) from FIG. 15A), or the voltagecomputed as the difference between the active and interphasemeasurements (e.g., V_(N3A)−V_(N3B) from FIGS. 15A and 15B). Normally,the algorithm would start with the compliance voltage, V+ at its maximumvalue (e.g., 16.8V), but could start at a lesser value.

Next, the minimum output voltage for the NDACs (Min(V_(NX))) isdetermined. This minimum output voltage would suggest the NDAC most atrisk to be in sub-saturation, and hence in this embodiment of thealgorithm is considered the most efficient to track. Accordingly, thealgorithm next asks how that minimum value compares relative to therange of guard band voltages for the NDACs. Essentially, if Min(V_(NX))is higher than the maximum guard band voltage for the NDACs (e.g.,1.8V), the compliance voltage V+ is decreased, because it can beinferred that all NDACs are at this point operating with output voltagesthat are too high to be optimal from a power consumption standpoint. Asshown, to expedite the iterative nature of the algorithm, the extent towhich the compliance voltage V+ is decreased scales with the extent towhich Min(V_(NX)) exceeds the upper guard band voltage for the NDACs.Thus, if Min(V_(NX)) is very high above the guard band (e.g.,Min(V_(NX))>4.8V) the compliance voltage is decreased by a large amount(e.g., 3.0V), but if barely above the guard band (1.8V<Min(V_(NX))<2.4V)the compliance voltage is decreased by a small amount (e.g., 0.6V).

Eventually, as the compliance voltage V+ is decreased, the minimumvoltage drop across the NDACs, Min(V_(NX)), will be within the guardband range (e.g., between 1.2V and 1.8V), and the PDACs can then beassessed. However, should Min(V_(NX)) fall below the minimum guard bandvoltage (e.g., 1.2V), the compliance voltage can be increased by anincrement (e.g., 0.6V prior to assessment of the PDACs. Of course, ifthe compliance voltage is at maximum at this point, further increasingthe compliance voltage will not be possible, and assessment of the PDACswill commence.

With the compliance voltage V+ adjusted at this point with respect tothe NDACs, the algorithm then acquires all relevant output voltages forthe PDACs (V_(P1), V_(P2), . . . V_(PY)). Again, the minimum voltagedrop across the PDACs, Min(V_(PY)) is determined, and the algorithm thenproceeds as with the NDACs. Specifically, if Min(V_(PY)) is above themaximum guard band voltages for the PDACS (e.g., 2.1V), the compliancevoltage is decreased, again by an amount commensurate with the deviationfrom the maximum guard band voltage.

Note that it is permissible to further decrease the compliance voltageat this point in the algorithm, even if some of the voltage drops acrossthe NDAC were close to the minimum guard band voltage. Thus, V+ can bedecreased if Min(V_(PY)) is above 2.1V for example, even whenMin(V_(NX)) is below 1.8V and otherwise is optimal. While this wouldseem to run the risk of adjusting the NDACs out of alignment, note thatMin(V_(NX)) is tied to (i.e., balanced with) Min(V_(PY)) by virtue ofthe current-voltage characteristics of both DACs. Because the currentsmust match for the NDACs and the PDACs, it is difficult to decreaseMin(V_(NX)) significantly below the minimum NDAC guard band threshold(e.g., 1.2V) without also bringing Min(V_(PY)) below the minimum PDACguard band voltage (e.g., 1.5V) and vice versa. Hence, due to thisbalancing between the NDACs and the PDACs, the compliance voltage can bereduced without significant risk of impacting circuit performance, i.e.,such that the circuitry is unable to produce an optimal current.

Eventually, as the compliance voltage is decreased, should the minimumoutput voltage across a PDAC be within the guard band for the PDACs(1.5<Min(V_(PY))<2.1, the compliance voltage is deemed optimal.Otherwise, should a PDAC be below the guard band at this point, thecompliance voltage can be increased by an increment (0.6V).

Thus, through the use of this exemplary algorithm, the compliancevoltage can be adjusted to an optimal value that is sufficient high toallow for proper circuit performance (i.e., sufficient current output),but low enough that power is not needlessly wasted via excessive voltagedrops across the current output circuitry. While the above specifies asingle embodiment of an algorithm for such dual current source and sinkoptimization, other algorithms are possible, and are subject to aprogrammer's particular preference, keeping power drain considerationand risk in mind. In short, the disclosed compliance voltage algorithmis merely representative of a manner for simultaneously optimizing boththe NDACs and the PDACs during actual stimulation.

It should be noted that while the measured output voltages across thesource circuit and the sink circuit are used to adjust the compliancevoltage in an attempt to bring both of the output voltages to a suitablelevel or levels, the technique can also be used to merely attempt tobring one of the output voltages to a suitable level (e.g., within arange, to a particular point, above or below a point, etc.). Thus, bothoutput voltages can be measured, and in another embodiment, benefits arehad by adjustment of the NDAC or PDAC output voltages alone, although ofcourse it is preferable to adjust both voltages.

It should be understood that the direction in which current flows is arelative concept, and different conventions can be used to definewhether currents flow to or from various sources. In this regard, arrowsshowing the directions of current flows in the Figures, references tocurrent flowing to or form various circuit nodes, references to currentsbeing sunk or sourced, etc., should all be understood as relatively andnot in any limiting sense.

It should also be understood that reference to an “electrode”implantable adjacent to a tissue to be stimulated includes electrodes onthe implantable stimulator device, or associated electrode leads, or anyother structure for directly or indirectly stimulating tissue.“Electrode” can also comprise a case electrode such as disclosed herein.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the literal and equivalent scope of the invention setforth in the claims.

What is claimed is:
 1. An implantable medical device, comprising: asource circuit coupled to a compliance voltage, wherein the sourcecircuit is configured to source a sequence of current pulses to at leasta first electrode; a sink circuit coupled to a reference voltage,wherein the sink circuit is configured to sink the sequence of currentpulses from at least a second electrode; voltage sensor circuitryconfigured to measure a first voltage across at least the source circuitand a second voltage across at least the sink circuit, wherein the firstand second voltages are measured between the current pulses when neitherof the source or sink circuits is activated to issue one of the currentpulses; and regulator circuitry configured to adjust the compliancevoltage in accordance with at least the first and second voltages. 2.The implantable medical device of claim 1, wherein the first and secondvoltages are measured simultaneously.
 3. The implantable medical deviceof claim 1, wherein the first and second voltages are measured atdifferent times.
 4. The implantable medical device of claim 3, whereinthe first and second voltages are measured between different of thecurrent pulses.
 5. The implantable medical device of claim 1, furthercomprising a switching matrix, wherein the switching matrix isconfigured to selectively couple the first and second voltages to thevoltage sensor circuitry.
 6. The implantable medical device of claim 1,further comprising a first switching matrix for coupling the sourcecircuit to the first electrode, and a second switching matrix forcoupling the sink circuit to the second electrode.
 7. The implantablemedical device of claim 1, further comprising a first switch forcoupling the source circuit to the first electrode, and a second switchfor coupling the sink circuit to the second electrode.
 8. Theimplantable medical device of claim 7, wherein the first voltagecomprises a voltage across the source circuit and the first switch, andwherein the second voltage comprises a voltage across the sink circuitand the second switch.
 9. The implantable medical device of claim 7,wherein the first voltage comprises a voltage across the source circuitbut not across the first switch, and wherein the second voltagecomprises a voltage across the sink circuit but not across the secondswitch.
 10. The implantable medical device of claim 1, wherein thesource and sink circuits comprise current mirrors.
 11. The implantablemedical device of claim 1, wherein the first voltage includes a voltagedrop across at least one first output transistor in the source circuit,and wherein the second voltage includes a voltage drop across at leastone second output transistor in the sink circuit.
 12. The implantablemedical device of claim 11, wherein the at least one first transistorcomprises a plurality of parallel-connected first transistors, andwherein the at least one second transistor comprises a plurality ofparallel-connected second transistors.
 13. The implantable medicaldevice of claim 11, wherein the regulator circuitry is configured toadjust the compliance voltage to change the first voltage such that theat least one first output transistor is in saturation, and to change thesecond voltage such that the at least one second output transistor is insaturation.
 14. The implantable medical device of claim 13, wherein theat least one first transistor comprises a plurality ofparallel-connected first transistors, and wherein the at least onesecond transistor comprises a plurality of parallel-connected secondtransistors.
 15. The implantable medical device of claim 1, furthercomprising a battery, wherein the regulator circuit derives thecompliance voltage from the battery.
 16. The implantable medical deviceof claim 1, wherein the voltage sensor circuitry is further configuredto measure a third voltage across at least the source circuit and afourth voltage across at least the sink circuit, wherein the third andfourth voltages are measured during at least one of the current pulseswhen the source and sink circuits are activated.
 17. The implantablemedical device of claim 16, wherein the third and fourth voltages aremeasured simultaneously.
 18. The implantable medical device of claim 16,wherein the third and fourth voltages are measured at different times.19. The implantable medical device of claim 18, wherein the third andfourth voltages are measured during different of the current pulses. 20.The implantable medical device of claim 16, wherein the regulatorcircuitry is configured to adjust the compliance voltage in accordancewith the first, second, third, and fourth voltages.
 21. The implantablemedical device of claim 20, wherein the regulator circuitry isconfigured to adjust the compliance voltage in accordance with a firstdifference between the third and first voltages, a second differencebetween the fourth and second voltages.
 22. The implantable medicaldevice of claim 21, wherein the regulator circuitry is configured toadjust the compliance voltage to reduce the first difference below afirst threshold, and to reduce the second difference below a secondthreshold.
 23. A method for adjusting a compliance voltage in astimulator device, comprising: using a source circuit and a sink circuitto provide a sequence of current pulses to a load between at least twoelectrodes; measuring a first voltage across at least the source circuitand a second voltage across at least the sink circuit during thestimulation pulses, wherein the first and second voltages are measuredbetween the current pulses when neither of the source or sink circuitsis providing one of the current pulses; and using at least the first andsecond voltages to adjust the compliance voltage, wherein the compliancevoltage is formed across the series connection of the source circuit,the load, and the sink circuit.
 24. The method of claim 23, wherein thefirst and second voltages are measured simultaneously.
 25. The method ofclaim 23, wherein the first and second voltages are measured atdifferent times.
 26. The method of claim 25, wherein the first andsecond voltages are measured between different of the current pulses.27. The method of claim 23, wherein the source and sink circuitscomprise current mirrors.
 28. The method of claim 23, wherein the firstvoltage includes a voltage drop across at least one first outputtransistor in the source circuit, and wherein the second voltageincludes a voltage drop across at least one second output transistor inthe sink circuit.
 29. The method of claim 28, wherein the at least onefirst transistor comprises a plurality of parallel-connected firsttransistors, and wherein the at least one second transistor comprises aplurality of parallel-connected second transistors.
 30. The method ofclaim 23, further comprising measuring a third voltage across at leastthe source circuit and a fourth voltage across at least the sink circuitduring the stimulation pulses, and using the first, second, third, andfourth voltages to adjust the compliance voltage.
 31. The method ofclaim 30, wherein the third and fourth voltages are measuredsimultaneously.
 32. The method of claim 30, wherein the third and fourthvoltages are measured at different times.
 33. The method of claim 32,wherein the third and fourth voltages are measured during different ofthe current pulses.
 34. The method of claim 30, wherein a firstdifference between the third and first voltages, a second differencebetween the fourth and second voltages, is used to adjust the compliancevoltage.